Because of linkage of MLC genes with disease susceptibility in multiple sclerosis, rheumatoid arthritis and probably other conditions, study of patients affords opportunity for defining MLC genes. In rheumatoid arthrtis almost a third of the patients are homozygous for MLC determinants. Lymphocytes from donors can be used as reference cells for MLC typing. The plan of procedure can be divided into 4 steps: 1. Random MLC among patients to identify non-stimulating donors; 2. Culture of non-stimulatory patients against each other to characterize specificities; 3. Survey of populations (patients and controls) for presence or absence of MLC determinants using reference cells obtained through previous steps; 4. Genotyping of subjects that carry identified LS determinants by reverse culture (reference cell responding). Studies of this type are proposed to extend preliminary observations in rheumatoid arthritis to a larger group of patients, to patients with the juvenile form of the disease and to patients with associated conditions such as Sjogren's syndrome. In addition it is proposed to examine similarly the following conditions: ankylosing spondylitis, systemic lupus erythematosus, chronic hepatitis and the carrier state for hepatitis B associated antigen (HBAg). These studies may shed light on the genetics of susceptibility to develop these conditions.